The invention relates to a particulate surface filter, regenerable by back-flushing, formed from a porous honeycomb monolith structure with selectively plugged passageways and microporous membrane coatings applied to the passageway surfaces.
Diesel particulate filters for removal of soot from combustion sources have been commercially available for several years, since the early 1980's. These devices are fabricated from a porous honeycomb ceramic monolith which contains a multiplicity of longitudinal passageways extending through the monolith between a pair of opposing end faces where the open cross sections of the passageways are exposed. The passageways themselves are formed by thin, porous walls which extend continuously between the end faces. The passageway densities can range from below 25 passageways per square inch to over 1000 passageways per square inch of cross-sectional area of the monolith. This monolith structure represents a highly-compact, high surface area filter.
The diesel filter is typically formed from such a monolith structure by plugging alternate ends of adjacent passageways. In this structure, exhaust gas flow is introduced into open passageways at the inlet face of the structure. These passageways are plugged at the downstream end face, and thereby gas flow is forced to flow through the porous walls of the monolith structure. Carbon soot is collected on and within the walls of the monolith structure. The soot so collected is removed by intermittent regeneration which is accomplished by thermal oxidation.
Unless the trapped, particulate matter is removed by combustion, it would be extremely difficult to regenerate a diesel filter. For example, regeneration by back flushing, a method frequently used for filters which function as surface filters, will be highly ineffective because particulate matter will have entered and plugged the pore structure. Filter structures which function in this manner, so called depth filters, are generally single use disposable filters.
The mean pore size of the ceramic materials used for honeycomb monoliths suitable for diesel particulate filters can vary, and is typically in the range of from about 10 microns to 50 microns. The pore size distribution of such materials is generally quite broad. A relatively large pore size is chosen so that the devices have a low pressure drop at the desired gas flow rate per unit filter area. The walls of such diesel filters have a pore structure which is substantially homogeneous across its thickness, and therefore, for the wall thickness typically employed, a finer pore size cannot be employed without creating a pressure drop undesirable for diesel applications.
The pore size and pore size distribution in diesel filters are such that if used in typical surface filtration applications, particulate matter would enter and clog the pore structure. Backflushing to regenerate the filter would be ineffective due to such clogging of the pore structure.